1. Field of the Invention
The present invention relates to a discharged sheet stacking apparatus for stacking discharged sheets, and, for example, is applied to an image forming apparatus such as an ink jet printer.
2. Related Background Art
In the past, as image forming apparatuses of this kind, there has been proposed ink jet printers for effecting printing by discharging liquid ink onto a printing medium (sheet).
In recent years, since the ink jet printer has advantage of high speed printing and high image quality, it has widely been used in a personal computer, a word processor and a facsimile.
By the way, in general ink jet printers, since the printing is effected by discharging the liquid ink onto the sheet, if an imaged surface of the sheet is contacted with any object or a human's finger before the ink printed on the surface of the sheet is dried, the imaged surface is contaminated.
To avoid this, in recent ink jet printers, a heater is incorporated into the printer to promote the drying of ink or there is provided a discharged sheet stacker (discharged sheet stacking apparatus) for preventing contamination of the imaged surface of the sheet by preventing a succeeding sheet from contacting with the imaged surface of a preceding sheet before the ink on the imaged surface is dried during the continuous printing.
Now, a conventional ink jet printer having such a discharged sheet stacker will be explained with reference to FIGS. 6 and 7.
FIG. 6 is a schematic perspective view of a conventional image forming apparatus (ink jet printer), and FIG. 7 is a schematic front view of the conventional image forming apparatus (ink jet printer).
As shown, in the printer 1, a paper sheet (sheet) 2 is conveyed by an internal sheet conveying mechanism (not shown) during a printing operation and is discharged from a discharge opening 3.
A sheet discharge roller (not shown) and a spur roller (not shown) are provided at the discharge opening 3, thereby feeding out the printed sheet toward a discharging direction while holding the sheet.
The sheet 2 is discharged along upper surfaces of two movable holding members or support members (right support member 4a and left support member 4b) disposed above a stacking reference surface (stacking portion) 5 which is disposed in front of the discharge opening 3 and on which the printed sheets are ultimately stacked.
Since the sheet 2 is held by the support members 4a, 4b until a time period required for drying ink on a preceding sheet 6 is elapsed, the sheet 2 is not contacted with the preceding sheet 6.
After the drying time period is elapsed, a means such as a cam shaft (not shown) for driving the support members 4a, 4b is operated to separate the support members 4a, 4b from each other by a distance greater than a width of the sheet, thereby dropping the printed sheet 2 onto the preceding sheet 6 rested on a discharged sheet stack portion 4.
In this case, it is important that the support members 4a, 4b are driven (opened) after the sheet 2 was completely fed out from the sheet discharge roller and the spur roller.
That is to say, if the sheet 2 is fed out from the sheet discharge roller and the spur roller after the support members 4a, 4b are opened, the succeeding sheet rubs the surface of the preceding sheet during the stacking operation or the sheet is not stacked horizontally.
In this way, after the time period (one sheet or more) required for drying the ink is surely elapsed, the sheet is discharged, thereby preventing the contact between the sheets.
Regrading a size of the sheet, a width of a sheet most available to the printer 1 is set as a maximum printing width of the printer and the discharged sheet stacking apparatus, and the support members 4a, 4b provided on the discharged sheet stacking apparatus are disposed in alignment with the size of the sheet having the maximum printing width.
When various sheets having different size are used, regardless of the size of the sheet, the sheets to be used are set in the printer 1 so that ends of the sheets abuts against a convey reference 7.
For example, in an example of the prior art, a width 8 of a sheet of fixed form (A4 size longitudinal) is 210 mm, and the support members 4a, 4b are disposed in alignment with this width.
Thus, in such a printer 1, when a fixed form sheet having a smaller width such as B5 size longitudinal (182 mm) or A5 size longitudinal (142 mm) is used, in the discharged sheet stacker, the sheet is held at only one side (support member 4a), i.e., the convey reference 7 side, and is floating at the other side. In the fixed form sheet, since the smaller the width the smaller a length, the sheet is held by inherent rigidity of the sheet itself.
However, in the above-mentioned conventional case, there arose the following problems.
As mentioned above, in the case where the sheet having small width is discharged, when the sheet is held only at one side in the width-wise direction, although the sheet can be held by its inherent rigidity, the other side of the sheet which is not held is suspended.
Thus, when the holding members (support members) are opened to release the holding condition, the sheet is dropped obliquely from the suspended side, and, it is difficult to control the dropping condition.
Accordingly, when a plurality of sheets are discharged continuously, the sheets are scattered at the stacking portion.
Thus, it is troublesome for the operator to collect the sheets.